Atrial tachyarrhythmias are quite common, and occur even in patients who are not acutely ill. One such atrial tachyarrhythmia, atrial fibrillation, does not usually pose an immediate threat to the patient's life, but since prolonged atrial fibrillation can compromise patient hemodynamics, its treatment is highly desirable. For patients having episodes of atrial fibrillation that do not respond to drug therapy, an electrical countershock delivered by an implantable cardioverter-defibrillator (ICD) may be used to convert the atrial tachyarrhythmia into a normal heart rhythm.
One method of delivering an electrical countershock to treat atrial fibrillation is illustrated in the Bardy U.S. Pat. No. 5,314,430. The Bardy patent describes a lead system having a coronary sinus/great vein electrode and a subcutaneous left pectoral plate electrode such as on a surface of a defibrillator housing. The Bardy lead system optionally includes a superior vena cava and/or ventricular electrode. The Bardy lead system can be employed in a device that is intended to perform atrial cardioversion or defibrillation only, or in a device that also performs ventricular cardioversion or defibrillation. However, placing the electrode in the coronary/sinus great vein portion of the heart is an extremely difficult procedure for the implanting physician. Moreover, in the case where both atrial cardioversion and ventricular defibrillation are desired, a separate ventricular lead is required. Therefore, there is a need in the art for a more convenient system for performing either atrial cardioversion or both atrial cardioversion and ventricular defibrillation.